Transit ridership fell in 31 of 35 major metropolitan areas in the United States last year, including the seven cities that serve the majority of riders, with losses largely stemming from buses but punctuated by reliability issues on systems such as Metro, according to an annual overview of public transit usage.

The analysis by the New York-based TransitCenter advocacy group, using data from the U.S. Department of Transportation’s National Transit Database, raises alarm about the state of “legacy” public transit systems in the Northeast and Midwest, and rising vehicle ownership and car-based commuting in cities nationwide.

Researchers concluded factors such as lower fuel costs, increased teleworking, higher car ownership and the rise of alternatives such as Uber and Lyft are pulling people off trains and buses at record levels.

The data also showed 2017 was the lowest year of overall transit ridership since 2005, and bus ridership alone fell 5 percent.

“I think it needs to be considered an emergency,” said Jarrett Walker, a transit planner who served as a consultant on a top-down bus network redesign to curb declining ridership in Houston. “When we don’t share space efficiently, we get in each other’s way. And that is a problem for the livelihood, the viability, the livability and the economy of a city . . . . It means more traffic, more congestion.”

The Washington region’s overall ridership fell in the middle of the pack with a 3.4 percent decline in overall trips between 2016 and 2017. Specifically, Metro’s ridership dropped by 3.2 percent. The trend was largely driven by a 6 percent decline in bus ridership. Dramatic losses to subway ridership, including a 10 percent decline in 2016, had appeared to level off by 2017, when the total number of trips fell by about a percent and a half.

Metro has said about 30 percent of its ridership losses are tied to reliability issues, with teleworking, a shrinking federal workforce, Uber and Lyft, and other factors to blame for the rest.

Exceptions to the trend: Seattle, Phoenix and Houston, which either expanded transit coverage and boosted service or underwent ambitious network overhauls, as in Houston’s case. (New Orleans ridership stayed flat.) In 2015, the Houston bus system was transformed overnight from a traditional hub-and-spoke design focused on downtown to a grid that apportioned equal service to other parts of the city. In the aftermath of the redesign, the system saw significant weekend ridership gains and quelled a trend of dramatic losses that included losing a fifth of its ridership over a little more than a decade.

That was not the case for the majority of U.S. cities. Between 2016 and 2017, ridership fell in each of the seven largest transit markets: New York, Chicago, Los Angeles, D.C., San Francisco, Boston and Philadelphia.

Transit researchers said it is crucial for cities and transit agencies to slow the losses even amid declining revenue, as alternatives threaten to lure people back into cars, particularly as shared rides become cheaper with the arrival of autonomous vehicles. The problem: The declines mean a decrease in farebox recovery, which can often lead to fare increases and reduced service, as in Metro’s case.

“The thing that’s perhaps a little bit more scary about this downturn [is] the prospect of technology will continue to nibble away [riders],” said Steven Polzin, program director for mobility policy research at the University of South Florida’s Center for Urban Transportation Research. He laid out the factors responsible: online shopping, distance learning, teleworking, ride-hailing apps and alternatives such as bike sharing.

Polzin described what he called a “tough political sell” for agencies faced with decreasing ridership.

“Ridership declines, and then fare revenue declines, and then you have to cut service which means ridership declines more,” he said. “So folks get nervous about the cyclical nature of the decline because of lost fare revenue. But [ridership declines] also undermine kind of the public will to invest additional subsidy dollars and service as well. It’s very hard to go to your government and say ‘My ridership is down 10 percent, and I need more money to subsidize 10 percent less riders.’ ”

Planners warn that cities simply do not have the capacity to handle a wholesale shift to other modes — whether today’s version of ride hailing, driving or eventual ride sharing through autonomous vehicles. Those alternatives, Walker said, are no match for “the basic geometry problem that only transit can solve — which is to move large numbers of people through a city in very little space.”

But some researchers said declining ridership is not always indicative of transit’s failures.

Los Angeles-area transit agencies have seen dramatic bus ridership declines since the mid-2000s, with overall bus ridership falling about 30 percent over the course of a decade, according to the Transit­Center analysis.

Michael Manville, an assistant professor of urban planning at the University of California at Los Angeles, co-authored a January 2018 study that found many of the losses could be attributed to increased car ownership, particularly among low-income and immigrant populations, who were in a better position to afford cars following the Great Recession.

“I think it puts transportation planners in a bit of an unusual position . . . if in fact the reason for that departure is low-income people are doing better, getting the ability to move around like everyone else, it’s hard to say that what we should do is get them to remove themselves from their cars and back on trains and buses,” Manville said. “Transit systems should deliver quality service to low-income people. But low-income people do not owe us a transit system.”

(Researchers also pointed out the increased ease of obtaining a car, through factors such as subprime auto loans.)

Walker warned of the future the trends could portend.

“That can’t just be a free-market­ conversation of transit losing ridership, that’s fine, let the best mode win,” he said. “City governments have an urgent imperative to do what’s necessary to make it attractive for people to use modes that use space efficiently.”

Metro and other systems’ reliability issues have hit low-income riders hardest, and now those systems are having a tough time winning them back in the face of increasing alternatives, advocates say.

Kristen Jeffers, founder and editor of the Black Urbanist blog, said riders are leaving because of declining service and the increased availability of other options to fill the gaps.

“Now that you have a car or a bike or a scooter on an app in your hand, and it’s right there — in a lot of major cities, why not use that?” Jeffers said. “Now you don’t have the indignity of being stuck on the side of the road for a bus that never comes.”

She said transit systems need to regain trust through community outreach and going out of their way to cater to riders who might previously not have had a choice.

“Treating the bus like a prestige system,” she said, similar to their treatment of heavy-rail systems in the past.

Metro is pondering a wholesale redesign of its bus system, with a study “to examine travel patterns, customer demand, technology opportunities and how to most cost-effectively deliver Metrobus service to riders,” according to agency spokeswoman Sherri Ly. The agency has yet to award a contract for the study, she said.

Meanwhile another West Coast city, Seattle, is viewed as the model for how transit agencies can recoup ridership in an era of population growth, an improving economy and rapid technological change — in part because of the popularity of buses. The city’s bus ridership has steadily grown from 92 million trips to 119 million over 16 years, the TransitCenter analysis shows. Meanwhile light-rail ridership ballooned amid expansions, from 3 million trips in 2002 to 32 million in 2017.

The city, which has some of the worst traffic congestion in the country, hosts about 45,000 Amazon employees and had added 60,000 workers to its center city core since 2010, according to Andrew Glass-Hastings, director of transit and mobility for the Seattle Department of Transportation.

Meanwhile Seattle voters have approved three high-dollar, ­transit-friendly initiatives that in the eyes of public officials have paid dividends and will continue to boost ridership: a $50 million annual funding boost to bus service, a billion-dollar Bus Rapid Transit expansion and a $54 billion light-rail expansion plan that would build 62 miles of light-rail in a project that will extend into the 2030s.

The improved bus service has meant the build-out of priority bus lanes and higher frequencies, with buses coming every four to six minutes, Glass-Hastings says. The state also requires large employers to enact programs that encourage alternatives to workers driving alone to work, resulting in ­commuter-benefit programs.

The lesson, says Glass-Hastings: “You can’t neglect your transit system for decades, have it be in disrepair and expect people to continue to use it, especially in a day and age when alternatives are so readily available.”

The Washington region, like many transit-centric cities, is a major player in the battle for Amazon’s second headquarters, which brings the promise of about 50,000 jobs. Glass-Hastings said HQ2 could be a coup for whichever city lands it. About 95 percent of workers to Seattle’s new center city jobs commute by a mode other than driving alone, he said, and in Amazon’s case, its workers’ transit costs are ­company-covered.

But Amazon’s preference of Seattle should be a message for the cities, he said:

“You can’t just drop 50,000 people in sort of a transit desert and expect them to seek out the bus.”